Arterial aneurysm

An arterial aneurysm is what happens when one stretch of an artery, one of the tubes that carry blood out of the heart under pressure, balloons outward at a specific spot and ends up wider than the rest. It isn't that the whole artery widens: it's a bulge in one localized place, and that place is where the wall of the tube has been weakened.

Picture an old garden hose that has worn thin at one point and swells into a pocket when the water runs through it. As long as the wall holds, the pocket just sits there. If that wall gives way and the aneurysm ruptures, blood pours out of the tube all at once, and that bleeding can cost a person their life.